Proximal spinal muscular atrophy (SMA) is an inherited, clinically heterogeneous group of neuromuscular disorders characterized by degeneration of the anterior horn cells of the spinal cord. Patients suffer from symmetrical weakness of trunk and limb muscles, the legs being more affected than the arms and the proximal muscles weaker than the distal ones; diaphragm, facial and ocular muscles are spared. There are three forms of childhood-onset SMA (types I, II and III), and a relatively recently categorized adult-onset form IV, all of which can be distinguished on the basis of age of onset and severity of the clinical course assessed by clinical examination, muscle biopsy and electromyography (EMG) (Munsat T L, Davies K E (1992)).
Type I (Werdnig-Hoffmann disease) is the most acute and severe form, with onset before six months and death usually before two years; children are never able to sit without support. Symptoms of the disease can be present in utero, as reduction of fetal movements; at birth; or more often, within the first four months of life. Affected infants are particularly floppy, experience feeding difficulties and diaphragmatic breathing, and are characterized by a general weakness in the intercostals and accessory respiratory muscles. Affected children never sit or stand and usually die before the age of 2; death is generally due to respiratory insufficiency.
Type II (intermediate, chronic form) has onset between six and eighteen months of age; muscular fasciculations are common, and tendon reflexes progressively reduce. Children are unable to stand or walk without aid. Feeding and swallowing problems are not usually present in Type II SMA, although in some patients a feeding tube may become necessary. Most patients generally develop a progressive muscular scoliosis which can require surgical correction. Like patients with type I disease, clearing of tracheal secretions and coughing might become difficult because of poor bulbar function and weak intercostal muscles. These patients have profound hypotonia, symmetrical flaccid paralysis, and no control of head movement.
Type III (Kugelberg-Welander disease, or Juvenile Spinal Muscular Atrophy) is a mild, chronic form, with onset after the age of 18 months; motor milestones achievement is normal, and deambulation can be preserved until variable ages. These patients often develop scoliosis, and symptoms of joint overuse, generally caused by weakness, are frequently seen. Life expectancy is almost normal but quality of life is markedly compromised.
Types I, II and III SMA progress over time, accompanied by deterioration of the patient's condition.
Adult-onset type IV is characterized by weakness in the second or third decade of life, with mild motor impairment not accompanied by respiratory or nutritional problems. Adult SMA is characterized by insidious onset and very slow progression. The bulbar muscles are rarely affected in Type IV. It is not clear that Type IV SMA is etiologically related to the Type I-III forms.
Other forms of spinal muscular atrophy include X-linked disease, spinal muscular atrophy with respiratory distress (SMARD), spinal and bulbar muscular atrophy (Kennedy's disease, or Bulbo-Spinal Muscular Atrophy), and distal spinal muscular atrophy.
SMA is due to mutations in the Survival of Motor Neuron (SMN) gene, which exists in two forms in humans (SMN1 and SMN2). Loss of SMN is deleterious to motor neurons and results in neuromuscular insufficiency, a hallmark of the disease. From a genetic point of view, SMA is an autosomal recessive condition, caused by disruption of SMN1 gene, located in 5q13 (Lefebvre S., et al. (1995) Cell 80: 155-165). More than 98% of patients with spinal muscular atrophy have a homozygous disruption of SMN1 by deletion, rearrangement, or mutation. All these patients, however, retain at least one copy of SMN2.
At the genomic level, only five nucleotides have been found that differentiate the SMN1 gene from the SMN2 gene. Furthermore, the two genes produce identical mRNAs, except for a silent nucleotide change in exon 7, i.e., a C→T change six base pairs inside exon 7 in SMN2. This mutation modulates the activity of an exon splicing enhancer (Lorson and Androphy (2000) Hum. Mol. Genet. 9:259-265). The result of this and the other nucleotide changes in the intronic and promoter regions is that most SMN2 are alternatively spliced, and their transcripts lack exons 3, 5, or 7. In contrast, the mRNA transcribed from the SMN1 gene is generally a full-length mRNA with only a small fraction of its transcripts spliced to remove exon 3, 5, or 7 (Gennarelli et al. (1995) Biochem. Biophys. Res. Commun. 213:342-348; Jong et al. (2000) J. Neurol. Sci. 173:147-153). All SMA subjects have at least one, and generally two to four copies of the SMN2 gene, which encodes the same protein as SMN1; however, the SMN2 gene produces predominantly truncated protein (SMNΔ7) and only low levels of full-length SMN protein.
The SMNΔ7 protein is non-functional and thought to be rapidly degraded. About 10% of SMN2 pre-mRNA is properly spliced and subsequently translated into full length SMN protein (FL-SMN), and the rest being the SMNΔ7 copy. The efficiency of SMN2 splicing might be dependent on severity of disease, and production of a full length transcript of SMN2 could range from 10% to 50%. Furthermore, presence or absence of the SMN1 gene, roughly 90% of which becomes the FL-SMN gene product and protein, influences the severity of SMA by whether or not it can compensate for the truncated SMNΔ7 copies. A low level of SMN protein allows embryonic development, but is not sufficient to sustain the survival of motor neurons of the spinal cord.
The clinical severity of SMA patients inversely correlates with the number of SMN2 genes and with the level of functional SMN protein produced (Lorson C L, et al. (1999) PNAS; 96:6307-6311) (Vitali T. et al. (1999) Hum Mol Genet; 8:2525-2532) (Brahe C. (2000) Neuromusc. Disord.; 10:274-275) (Feldkotter M, et al. (2002) Am J Hum Genet; 70:358-368) (Lefebvre S, et al. (1997) Nature Genet; 16:265-269) (Coovert D D, et al. (1997) Hum Mol Genet; 6:1205-1214) (Patrizi A L, et al. (1999) Eur J Hum Genet; 7:301-309).
Current therapeutic strategies for SMA are mostly centered on elevating full length (wild type) SMN protein levels, modulating splicing towards exon 7 inclusion, stabilizing the wild type protein, and to a lesser extent, on restoring muscle function in SMA by providing trophic support or by inhibiting skeletal muscle atrophy.
The mechanism leading to motorneuron loss and to muscular atrophy still remains obscure, although the availability of animal models of the disease is rapidly increasing knowledge in this field (Frugier T, et al. (2000) Hum Mol. Genet. 9:849-58; Monani U R, et al. (2000) Hum Mol Genet 9:333-9; Hsieh-Li H M, et al. (2000) Nat Genet 24:66-70; Jablonka S, et al. (2000) Hum Mol. Genet. 9:341-6). Also the function of SMN protein is still partially unknown, and studies indicate that it can be involved in mRNA metabolism (Meister G, et al. (2002). Trends Cell Biol. 12:472-8; Pellizzoni L, et al. (2002). Science. 298: 1775-9), and probably in transport of proteins/mRNA to neuromuscular junctions (Ci-fuentes-Diaz C, et al. (2002) Hum Mol. Genet. 11: 1439-47; Chan Y B, et al. (2003) Hum Mol. Genet. 12:1367-76; McWhorter M L, et al. (2003) J. Cell Biol. 162:919-31; Rossoll W, et al. (2003) J. Cell Biol. 163:801-812).
In addition to the SMAs, a subclass of neurogenic-type arthrogryposis multiplex congenita (congenital AMC) has separately been reported to involve SMN1 gene deletion, suggesting that some degree of pathology in those afflicted is likely due to low levels of motor neuron SMN. (L. Burgien et al., (1996) J. Clin. Invest. 98(5):1130-32. Congenital AMC affects humans and animals, e.g., horses, cattle, sheep, goats, pigs, dogs, and cats. (M. Longeri et al., (2003) Genet. Sel. Evol. 35:S167-S175). Also, the risk of development or the severity of amyotrophic lateral sclerosis (ALS) has been found to be correlated with low levels of motor neuron SMN.
There is no cure or effective treatment for SMA available to date and therefore it would be advantageous to provide novel methods for modulating SMN in order to treat those afflicted with SMA, with neurogenic congenital AMC, ALS, or with other SMN-deficiency-related conditions. It would further be advantageous to provide novel drug targets that could be used as a basis for developing effective therapeutics or diagnostics for such neuronal conditions.